1. Field of the Invention
The invention relates to methods for manufacturing a semiconductor device with improved device performance and a device structure exhibiting the improved device performance, and more particularly to a method for manufacturing a SiGe-based device exhibiting reduced diffusion of an N type impurity.
2. Background of the Invention
The escalating requirements for ultra large scale integration semiconductor devices require ever increasing high performance and density of transistors. With scaling-down of a device dimension reaching limits, the trend has been to seek new materials and methods that enhance device performance. One of the most preferred methods is through mobility enhancement.
It is known that biaxial tensile strain applied to a channel region increases electron mobility in NFET devices. This can be achieved by building an NFET device, which is composed of a set of stacked films (e.g., silicon-SiGe-silicon) on a substrate. Starting from a silicon substrate, SiGe is grown on the silicon substrate. Buffered layers have typically been used to reduce threading dislocation defect density that can affect device leakage but still achieve enough relaxation through misfit dislocation formation. The SiGe film is relaxed so that it has a lattice constant larger than that of silicon. When the silicon is then deposited on the SiGe, it conforms to the larger lattice of the relaxed SiGe and undergoes biaxial tension. The channel is fully contained within this strained silicon and the electron mobility is enhanced.
The SiGe-based substrate, however, exhibits certain drawbacks, especially when NFET devices are formed thereon. To form an NFET device, an N type impurity (e.g., As or P) is ion-implanted onto the SiGe-based substrate to form active regions (e.g., source and drain regions). Here, the excessive amount of vacancies contained in the SiGe layer undesirably increases diffusion of the implanted N type impurity. This makes it more difficult to achieve consistent roll-off device characteristics. Therefore, there is a need for effective methodology for manufacturing a SiGe-based semiconductor device.